Slips are used for various downhole tools, such as bridge plugs and packers. The slips can have inserts or buttons to grip the inner wall of a casing or tubular. Examples of downhole tools with slips and inserts are disclosed in U.S. Pat. Nos. 6,976,534 and 8,047,279. Inserts for slips are typically made from cast or forged metal, which is then machined and heat-treated to the proper engineering specifications according to conventional practices.
Inserts for slips on metallic and non-metallic tools must be able to engage with the casing to stop the tool from moving during operation. On non-metallic tools, the inserts can cause the non-metallic slips to fail when increased loads are applied. Of course, when the slip fails, it disengages from the casing.
When conventional inserts are used in non-metallic slips, they are arranged and oriented as shown in FIG. 1A. The slip 20 is disposed adjacent a mandrel 10 of a downhole tool, such as a bridge plug, packer, or the like. The slip 20 moves away from the mandrel 10 and engages against a surrounding tubular or casing wall when the slip 20 and a cone 12 are moved toward one another. Either the slip 20 is pushed against the ramped surface 13 of the cone 12, the cone 12 is pushed under the incline 23 of the slip 20, or both.
As shown in FIG. 1A, the pockets 22 and the inserts 24 disposed in those pockets 22 intersect the slip 20 at an acute bite angle β with respect to a line perpendicular to the slip's surface 21. Thus, the conventional arrangement places the inserts 24 at an angle β toward the ramped surface 13 of the cone 12 and the incline 23 of the slip 20. The angle β can be from 10 to 20-degrees, for example, so that the top face of the insert 20 is oriented at the same angle β relative to the top surface of the slip 20.
By providing this angle β, the inserts 24 can better engage the casing C. For example, when the slip 20 is fully extended to a set position against the casing wall, the inserts 24 inclined by the acute angle β present cutting edges with respect to the inside surface of the casing C. With this arrangement, the inserts 24 can penetrate radially into the casing C. Angled toward the cones 12, this penetration can provide a secure hold-down against pushing and pulling forces that may be applied through the tool's mandrel 10 and element system.
The arrangement of the inserts 24, however, can damage the slips 20 or the inserts 24 themselves. As shown in FIG. 1B, load on the cone 12 during use of the downhole tool can cause the inserts 24 to put stress on the slip 20. As a result, the slip 20 can fracture at the edges of the pockets 22 toward the top surface 21 and the bottom surfaces 27 and 23 of the slip 20. In another form of failure shown in FIG. 1C, shear forces on the inserts 24 can cause the exposed ends of the inserts 24 to shear off along the slip's top surface 21.
The inserts 24 are typically composed of carbide, which is a dense and heavy material. When the downhole tool having the slips 20 with the carbide inserts 24 are milled out of the casing C, the inserts 24 tend to collect in the casing C and are hard to float back to the surface. In fact, in horizontal wells, the carbide inserts 24 may tend to collect at the heel of the horizontal section and cause potential problems for operations. Given that a well may have upwards of forty or fifty bridge plugs used during operations that are later milled out, a considerable number of the carbide inserts 24 from the milled plugs may be left in the casing and difficult to remove from downhole.
As noted previously, the small button inserts 24 create high stress points in the slips 20. This high stress is caused by the point loading on the edges of the inserts 24 or by a high stress across the cross-section of the inserts 24. During use then, the high stress points cause the inserts 24 to pitch, roll, and or depress in the slip 20. This can sometimes cause catastrophic failures of the slip's material, which can be metal, composite, plastic, etc.
Typically, to reduce the stress on the inserts 24, the cone and ramp angles can be adjusted to vary the radial load. The lengths of the inserts 24 as well as their angles in the slips 20 have also been adjusted. For instance, the angle of the inserts 24 has been adjusted both about the center plane of the slip 20 as well as the front plane of the slip 20 (either side-to-side or front-to-back). Some different angular arrangements for the inserts in the slips according to the prior art are discussed below.
FIGS. 2A-2B illustrate a side cross-section and end view of a slip 40 having a first arrangement of holes 46, 48, and 50 for inserts 60 according to the prior art. The slip segment 40 has first and second ends 42 and 44, which may be referred to as abutment end 42 and free end 44. An inner surface 41′ preferably has a shape complementary to the outermost surface of a mandrel (not shown) to which the slip segment 40 is mounted. The slip segment 40 also has first and second sides 43 and 43′ and has a forward or outer arcuate face 41. The free end 44 has an incline 44′ on the inner surface 41′.
A plurality of buttons or inserts 60 are secured to the slip segment 40 and extend externally outwardly from the outer arcuate surface 41. They are secured in cavities defined in the slip segment 40. The cavities may be referred to as first, second and third cavities 46, 48, and 50 with longitudinal central axes 45, 47, and 49, respectively. As best shown in FIG. 2B, the cavities 46, 48, and 50 are oriented so that the longitudinal axes 45, 47, and 49 lie in intersecting vertical planes. As best shown in FIG. 2A, each of the longitudinal central axes 45, 47, and 49 can be angled from a horizontal axis by an angle θ, which may be, for example, approximately 15-degrees.
FIGS. 3A-3B illustrate a side cross-section and end view of a slip 40 having a second arrangement of holes 46, 48, and 50 for inserts 60 according to the prior art. As before, the slip segment 40 has first and second ends 42 and 44, which may be referred to as abutment end 42 and free end 44. The slip segment 40 has first and second sides 43 and 43′ and has a forward or outer arcuate face 41. An arcuate inner surface 41′ preferably conforms to the shape of the outer surface of a mandrel against which the slip segment 40 disposes. Finally, the free end 44 has an incline 44′ on the inner surface 41′.
Buttons or inserts 60 are secured to the slip segment 40 and extend outwardly from outer arcuate face 41. The inserts 60 are secured in cavities, which include first, second and third cavities 46, 48, and 50. The cavities 46, 48, and 50 have longitudinal axes, identified as longitudinal axes 45, 47, and 49, respectively. The inserts 60 are preferably cylindrically shaped buttons with longitudinal central axes. The longitudinal axes 45, 47, and 49 are parallel, and as such, the longitudinal central axes of the inserts 60 in the slip segment 40 are parallel to one another. As best shown in FIG. 3A, each of longitudinal central axes 45, 47, and 49 can be angled from a horizontal axis by an angle θ, which may be, for example, approximately 15-degrees.
Although various arrangements of inserts in slip segments have been suggested in the past, operators are continually striving to use new materials, different load distributions, and the like to meet new challenges in the downhole environments.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.